The invention described herein relates to dynamoelectric machines and more particularly to an improved damper shield used with superconducting synchronous motors or generators.
The field windings of superconducting electric machines conventionally are mounted on the machine rotor, and are enclosed in both a concentrically disposed radiation shield and a damper shield. These rotating shield elements are joined to form part of a dewar system which maintains the field winding under a vacuum at 4.2.degree. K (-452.degree. F), to achieve unusually high machine efficiency and performance. The radiation shield which includes both radial and axial members is placed between the warm and cold portions of the machine to reduce the direct thermal radiation from the stator and other supporting structure operating at ambient temperature to the lower temperature components on the rotor. The damper shield concentrically mounted outwardly from the radiation shield and is normally maintained at or near room temperature. It serves the dual function of providing restoring torque to the rotor as a result of load changes, and of shielding the field winding from ac fields generated in the stator from penetrating the low temperature 4.2.degree. K zone, during steady state and transient conditions to minimize undesirable losses. Since the costs for removing these losses by circulating liquid helium through the machine are relatively high, the damper shield is made of a material having high electrical conductivity to provide the protective screening function.
Under circumstances of a terminal short circuit on a superconducting three phase machine, the damper shield can be subjected to radially crushing forces which results from the interaction of armature flux and the shielding flux generated by current induced in the shield. These forces which may act on the shield are illustrated in FIG. 1 and represent those forces resulting from a full three phase short circuit. The forces P.sub.1 and P.sub.2 for simplicity purposes, are shown as concentrated forces although it will be understood such forces are actually forces distributed around the rotor that vary as sin.sup.2 .theta.. As shown, force P.sub.1 remains fixed with respect to the rotor surface while force P.sub.2 moves around the rotor at twice synchronous speed. The combined effect of forces P.sub.1, P.sub.2 create radially directed crushing forces which may reach values as high as 5000 psi, and cause the damper shield and its support structures to deflect and create high bending stresses in both the support and cylindrical structure. The most important disadvantage resulting from structural deformation is that the deflection could result in contact between the low temperature radiation shield and the field winding which will introduce thermal losses that could result in the loss of superconductivity. Such loss of superconductivity would render the generator useless from both efficiency and performance standpoints. The high stresses in the support structure could also result in plastic deformation that may result in complete structural failure with consequent excessive damage to the machine.
In addition to the radial crushing forces P.sub.1, P.sub.2 discussed above, a torque T, is also developed in the shield that varies as a damped sine wave following the fault. Depending on the machine stability, such peak torques as may be developed can be as high as 10 times rated machine torque. Since these high oscillating torque values can be reached, a stronger, heavier drive shaft for the machine is required to prevent damage during fault conditions.